Carbon fibers having an internal cavity

ABSTRACT

A reinforcing fiber is provided comprising a carbon fiber having a length and outer and inner surfaces defining a wall. The wall has a first cross section which defines an outer diameter and an inner diameter and a wall thickness. The wall thickness of the cylindrical carbon fiber tube is greater than or equal to 2 um.

TECHNICAL FIELD

The present disclosure relates to carbon fibers having an internal cavity and methods of producing the same.

BACKGROUND

Composite panels are commonly used to manufacture structural and body panels for vehicles and in other products. Composite panels are typically made of polymeric resins that are reinforced with carbon fibers, glass fibers, natural fibers, or the like which are dispersed in the matrix. Composite panels are typically strong, light weight and may be used in a wide variety of product applications.

SUMMARY

According to one aspect of this disclosure, a reinforcing fiber is provided comprising a carbon fiber having a length and outer and inner surfaces defining a wall. The wall has a first cross section which defines an outer diameter and an inner diameter and a wall thickness. The wall thickness of the cylindrical carbon fiber tube is greater than or equal to 2 um.

According to other aspects of this disclosure, the present invention includes a reinforcing fiber comprising a carbon fiber having a length and outer and inner surfaces defining a tube and a first cross section defining an outer diameter and an inner diameter wherein the outer diameter and the inner diameter are varying along the length.

According to another aspect of this disclosure, a method is provided for forming a polymer precursor from a polymer material. The polymer precursor has a length and outer and inner surfaces defining a cylindrical tube. The tube has a cross section defining an outer and inner diameter and a wall thickness. The wall thickness is greater than or equal to 2 um.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary perspective view of a carbon fiber having an internal cavity;

FIGS. 2A, 2B and 2C are fragmentary perspective views of a carbon fiber having an internal cavity, each view duplicating a shaping variation;

FIGS. 3A and 3B are a perspective view associated with a method for making polymer precursors for carbon fibers having internal cavities;

FIG. 4A is a perspective view associated with another method for making polymer precursors for carbon fibers having internal cavities;

FIG. 4B is a perspective view of comb-like micro pins used in the method of making polymer precursors of FIG. 4A;

FIG. 4C is a side view of a tooling plate associated with the method of making polymer precursors for of FIG. 4A;

FIG. 5A is a cross section view of a gas delivery tube for a method of making polymer precursors from a liquid polymer material; and

FIG. 5B is an end view of a bushing for gas delivery tubes as shown in FIG. 5A.

FIGS. 6A and 6B are a schematic of the steps in the method of making the carbon fibers having an internal cavity.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.

Carbon fiber (CF) reinforced polymeric composites are gaining increasing interest in the automotive industry as a promising light weight material to meet governmental corporate average fuel economy (CAFE) requirements and to meet customer expectations for fuel economy. To meet the economical requirements of high volume production of automotive composites, low cost manufacturing processes and low cost materials are being developed.

Incorporation of carbon fiber into composite structures has been met with challenges because carbon fiber process methods are labor intensive and yield porous carbon fibers and are not suited to automotive production volumes. Such labor intensive process methods include vacuum bag autoclaving of pre-impregnated carbon fiber composite laminates. Some attempts have been made to adapt the processing methods of composites that were developed around glass fiber reinforcements to that of carbon fiber reinforcements. These attempts have been met with challenges. The diameter of carbon fiber is typically half that of glass fiber. Accordingly, for an equivalent fiber volume loading, four times as many carbon fibers may be required to fill the same volume as compared to when using glass fiber. Particularly for random fiber composites, an increase in fiber quantity adds complexity to chopping processes due to the intimate interaction of the fibers and sizing formulation (thin layer of polymer coating) developed for carbon fibers. This fiber interaction may make the fibers clump during processing and result in inadequate dispersion of fibers. This will cause degradation in load transfer of the fibers and greatly reduce the composite mechanical properties. Hollow carbon fibers produced by a partial sulfonation process have graphene structure only near the fiber outer surface. Most of the content of the fibers produced by sulfonation are amorphous carbon, and are porous. This low crystallinity and high porosity may lead to lower strength and modulus than required for carbon reinforcing fibers.

Referring now to FIG. 1, the hollow carbon fiber 10 (having an internal cavity or void 11) has been formed as a cylinder tube having an outer diameter 12 and wall thickness 14 defining a cross sectional area 16. The outer diameter 12 of the fiber 10 may be from 7 μm to 30 μm, preferably from 10 to 20 um and most preferably from 13 to 15 um. Both the outer diameter 12 and the cross sectional area 16 can be adjusted and may vary along the length of the tube. The cross sectional area 16 may be from 30% to 80% of the total fiber cross sectional area. The total fiber cross sectional area is π(d/2)² where d is the outer diameter 12. The wall thickness 14 may be from 1 um to 10 um, preferably from 2 um to 5 um and most preferably from 2 um to 4 um. The outer diameter 12 of the hollow carbon fiber 10 is selected to address the issue of fiber dispersion and wet out in random fiber processing. Carbon fibers using the dimensions set forth in one or more embodiments results in a low density of fibers for reinforcement in polymeric resins. (Fibers 10, and fewer fibers required for a volume of polymeric resin, results in less interaction of the fibers and clumping.) The design having an internal cavity, sometimes referred to as a hollow core design reduces the oxidation and diffusion pathway within the polymer precursor tube thus keeping approximately the same stabilization and oxidation time as current CF manufacturing for fibers that are approximately 7 um in diameter. One or more embodiments provide carbon fibers 10 that can be produced with a diameter similar to glass fiber. The fibers of one or more embodiments may have a diameter that is 2× or greater than standard carbon fibers. The fibers of one or more embodiments of this disclosure may have a crystalline or graphene structure and are non-porous. High crystallinity and low porosity results in good mechanical tensile strength and tensile modulus. Specific tensile strength is the tensile strength divided by density and acceleration of gravity (g).

${{specific}\mspace{14mu} {tensile}\mspace{14mu} {strength}} = {\frac{{tensile}\mspace{14mu} {strength}}{{density} \times {acceleration}\mspace{14mu} {of}\mspace{14mu} {gravity}} = {\frac{\frac{Newton}{{Meter}^{2}}}{\frac{Kilograms}{{Meter}^{3}} \times \frac{newton}{Kilogram}} = {meter}}}$

The specific tensile strength for the fibers of this disclosure may range from 5×10⁴ m to 50×10⁴ m, more preferably from 10×10⁴ m to 40×10⁴ m, and most preferably 20×10⁴ m to 30×10⁴ m. The specific tensile modulus is the tensile modulus divided by density and acceleration of gravity (g). The specific tensile modulus for the fibers of this disclosure may range from 5×10⁶ m to 20×10⁶ m, and more preferably from 10×10⁶ m to 18×10⁶ m, and most preferably 12×10⁶ m to 15×10⁶ m.

One or more embodiments provide a relatively low cost material and manufacturing process for carbon fiber reinforced materials that are crystalline and are not porous. Other embodiments of this disclosure may provide a hollow carbon fiber that is a non-cylindrical shape, for example a shape having a square or oblong cross section, or any other suitable profile.

The outer and inner diameters 12 and 18 of the hollow carbon fibers 10 may vary along the length. The wall thickness 14 and cross sectional area 16 may also vary. Referring to FIG. 2A, the outer diameter 12 of the hollow carbon fiber 20 varies linearly along the length. The inner diameter 18 may also vary to maintain essentially the same cross sectional area 16 or it may remain the same, or it may vary in a manner different from the outer diameter 18. Referring to FIG. 2B, a carbon fiber tube 22 is shown where the inner diameter 18 and the outer diameter 12 vary and the cross sectional area 16 remains the same along the length of the carbon fiber 22. Referring to FIG. 2C, a carbon fiber tube 24 is shown where the outer diameter 12 and the inner diameter 18 vary together in an accordion style.

The carbon fiber may be in a shape defining a hollow structure other than a tube or cylinder. The cross section of the hollow structure may be a square, oblong, rectangular or other shape. A first cross section taken at a first position along the length of the hollow carbon fiber and a second cross section taken at a second position along the length have cross sectional areas that are substantially the same or may vary 80%, 50%, 20%, 6% or 0.5%.

CFs are manufactured from their polymer precursors via a series of tensioning, stabilization, carbonization processing etc. The precursor shrinks over these processing by about half. One or more embodiments provides CF precursors that have the same hollow design but with all the dimensions doubled. The benefits of this design include material savings and lower fiber density. The hollow core design can save a substantial amount of material and make the fiber even lighter.

One or more embodiments involve different manufacturing methods for producing the hollow polymer precursor for the hollow carbon fiber. The embodiments may be continuous processes so as to meet the demand of high volume manufacturing for automotive and other applications. Once the polymer precursor is formed, the hollow polymer precursor is oxidized and stabilized at 200° C. to 300° C. for ˜2 hours at atmospheric pressure. The polymer precursor is then carbonized at 1200° C. to 2900° C. depending on the grade of the carbon fiber. The diameter of the polymer precursor decreases during the carbonization process. The outer diameter of the polymer precursor may vary from 100 um to 10 um to form the hollow carbon fiber.

FIGS. 3A and 3B depict a perspective view associated with the initial step of forming polymer precursors from polymer materials in a sheet or film form. Polymer material 30 is produced in a sheet or film from CF precursor pellets. The polymer material 30 is pulled against and across one or more tooling plates 32. The tooling plates have a series of half circle features 34 extending in a longitudinal direction where the features transition to a flat shape. The film initially approaches the tool at the flat end and is tensioned and gradually formed into corrugated half-tube structures as it is pulled. The film direction may change from 10 degrees to 90 degrees from where the film touches the tool plate 36 to the end of the tool plate 38. Two half-tube films are finally hot pressed together to form complete tubes and split into individual hollow filaments. The hollow polymer precursor tubes may be collected by a spool 40.

Referring now to FIG. 4A, a method of direct die casting of hollow tubes is provided. Polymer material 42 is heated to become re-formable and then pressed together by one or more tooling plates 32 with the half circle features 34. Comb-like micro pins 44, shown in FIG. 4B, are placed at the center of the tooling plate 32 to ensure that a hollow polymer precursor is formed. The hollow fibers are formed and may be collected by a spool. The films may be continuously pressed. The polymer material 42 and the polymer precursor may be pulled across the tooling plate 32. Referring to FIG. 4C, a cross section of the tooling plate 32 with half circle features 34 and comb-like micro pins 44 is shown.

Referring now to FIGS. 5A and 5B, a method of making a polymer precursor is provided from a polymer material that flows and can be pumped. It may be a liquid, in a solution or as pellets. Carbon fiber materials are melted or dissolved in solutions and pumped through a bushing 50. An air, or gas, delivery tube 52 in the center of bushing holes 54 make the polymer precursors form a hollow tube structure after they are drawn out from the bushing 50 and solidified. The delivery tube prevents the hollow polymer precursor tube wall from collapse and can be a hollow tube or manifold or can be made from a porous steal rod. It can be used with or without gas flowing. It can also be a solid tube. When gas is introduced through the gas delivery tube 52, it flows out of a manifold outlet 56.

The method of forming the polymer precursor for the hollow carbon fiber may utilize mating of two sections, or partial tubes, having unequal size. A polymer precursor is formed on a tooling plate sized to produce a portion of the polymer precursor having a cross section that is more than half the cross section to be formed, with a portion that is less than half of the final cross section. The complete cross sectional shape is then formed by joining partial tubes that are not each half of the carbon fiber to be formed. The method of forming the carbon fiber may include tooling plates and bushings shaped to produce polymer precursors of different cross sectional shapes such as square or rectangular hollow fibers.

Referring now to FIGS. 6A and 6B, a flow chart of the steps in the method of making hollow carbon fibers is provided. A polymer material is provided in step 1 in the form of a sheet 60, a liquid 62, a solution or pellets. A tooling plate component is provided in step 2 with features 64 on one or both sides. The features may be half round or a portion of the final tube to be formed. The features may be flat on an end of the tooling plate and transition to the desired shape at a second end of the tooling plate. Alternatively the tooling plate component may be shaped with holes 54 or openings to provide the features by having the polymer material provided in step 1 be a material which is a liquid 62 and flows through the openings. In step 3, portions of the polymer precursor are joined by hot pressing to form the final polymer precursor shape with walls. In step 4, shaped walls that are connected, are split into discrete polymer precursors. Step 3 and step 4 would not be required for the tooling plate component designed for a polymer material that flows. If desired, the polymer precursor can be wound onto spools in step 5. Winding on spools provides a method of transporting and feeding the polymer precursor in the next step, step 6, where the polymer precursor is oxidized and stabilized. Oxidation in step 6 may be done at atmospheric pressure and 200-300° C. for approximately two hours. The oxidized and stabilized polymer precursor is then carbonized in step 7 at 1200° C. to 2900° C. The required temperature depends on the quality of the polymer precursor used. Finally, in step 8 a non-porous, crystalline hollow carbon fiber 10 is formed that has a smaller diameter than the polymer precursor. The fiber diameter is reduced in steps 6 and 7 and may decrease in size by up to a factor of two.

While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention. 

What is claimed is:
 1. A reinforcing fiber comprising: a carbon fiber having a length and outer and inner surfaces defining a wall having a first cross section defining an outer diameter and an inner diameter and a wall thickness, the wall thickness being greater than or equal to 2 um.
 2. The reinforcing fiber of claim 1, wherein the specific tensile strength of the carbon fiber is greater than 5×10⁴ meters.
 3. The reinforcing fibers of claim 1, wherein the wall is a cylindrical wall.
 4. The reinforcing fiber of claim 1, further comprising a second cross section taken at a second position along the length and having a cross sectional area substantially equal to that of the first cross section.
 5. The reinforcing fiber of claim 1, further comprising a second cross section taken at a second position along the length and having a cross sectional area that varies by less than 50% as that of the area of the first cross section.
 6. The reinforcing fiber of claim 1, wherein the outer diameter is greater than 7 um.
 7. The reinforcing fiber of claim 1, wherein the inner diameter is greater than 4 um.
 8. The reinforcing fibers of claim 1, wherein the carbon fiber further comprises first and second open ends.
 9. The reinforcing fiber of claim 1, wherein the carbon fiber further comprises a second cross section taken at a second position along the length, wherein the inner or outer diameters of the cross section vary by less than 5 um.
 10. A reinforcing fiber comprising: a carbon fiber having a length and outer and inner surfaces defining a tube and a first cross section defining an outer diameter and an inner diameter wherein the outer diameter and the inner diameter varying along the length.
 11. The reinforcing fiber of claim 10 wherein the outer diameter and the inner diameter increase linearly along the length.
 12. The reinforcing fiber of claim 10 wherein the outer diameter and the inner diameter increase and decrease repeatedly along the length.
 13. A method comprising: forming a polymer precursor from a polymer material, the polymer precursor having a length and outer and inner surfaces defining a cylindrical tube having a first cross section defining an outer and inner diameter and a wall thickness, the wall thickness being greater than or equal to 2 um.
 14. The method of claim 13 wherein the polymer material is in sheet form.
 15. The method of claim 13, wherein the polymer material is a polymer material that flows; heating a first film and a second film to become formable; pressing the first film and the second film together onto plates having half round channel having pins through a center of the half round channel; and pulling the first film and the second film in a longitudinal direction across the half round channels with gas flowing through the pins to form a hollow filaments.
 16. The method of claim 13, wherein the polymer material is a first and second precursor film; pumping liquid precursors through a bushing in a longitudinal direction, the bushing having a gaseous delivery tube; and flowing gases through the delivery tube to form a hollow filament.
 17. The method of claim 13, wherein the polymer material is a first and second precursor film; feeding the first film and the second film against a curved tool plate; pulling the first film and the second film across the curved tool plate in a longitudinal direction to form two partial tubes; mating the first film to the second film with the two tubes adjacent and longitudinally aligned; pressing the two tubes to form a series of tubes; and splitting the tubes into an individual hollow filaments.
 18. The method of claim 17, further comprising winding the hollow filaments on spools.
 19. The method of claim 17, wherein the feeding of the first film and the second film is more than 15 degrees from an initial direction.
 20. The method of claim 17, further comprising winding the hollow filaments on spools. 